In most electrical protection applications, it is essential for the protecting device to switch to a state where current flow into the protected equipment is substantially blocked or limited. This general observation holds for various protection approaches, such as fuses, circuit breakers, and transient blocking units (TBUs).
Current limiters can be utilized when the resulting power dissipation during a fault protection condition is limited to a level that is insufficient to cause damage to the current limiting device.
In VDSL applications 100, such as in FIG. 1a, the driver 102 is AC coupled by virtue of the transformer, and hence long term short circuit currents at the line side cannot cause sustained DC currents in the driver. A current limiting protection device can therefore be advantageously used to protect the driver.
Whereas a TBU will drastically reduce the output current when the current rises above a certain threshold, a current limiter will limit the current at a particular level, thus preventing excessive currents in the driver in the event of a transient. As the transient is short lived, by virtue of the AC coupling, a sustained fault due to lightning or power cross cannot cause problems of over dissipation in the current limiter.
FIG. 1b shows a graph of a generally poor linearity of a prior art current limiting device, where it is known that a changing gate drive to both the forward and reversed transistor elements causes modulation of the transistor resistances. Consequently, the linearity error may become significant at currents above about 50% of the required limiting threshold.
What is needed is a current limiting device that provides area savings, cost reduction and improvement in resistance compared to the use of a standard TBU. Further, there is a need for a solution for protecting low voltage drivers in the VDSL application for example, where low resistance, good linearity and fast response is key.